In recent years, display devices including thin-film transistors have been put to practical use. Examples of the display devices are a liquid crystal display device and an organic electroluminescence display device. With an increasing demand for higher definition of the display device, the area of a pixel aperture portion, which contributes to display, has become smaller. The size of the thin-film transistor, which is disposed in each pixel, tends to become smaller, in order to suppress a decrease in area of the pixel aperture portion. However, the reduction in size of the thin-film transistor is a factor of an increase in leak current.
When the leak current in the thin-film transistor is large, there is a concern that such a display defect that a pixel becomes a bright point will occur. As a structure of a thin-film transistor for suppressing a leak current, there is known such a structure that a low-concentration impurity region, or a so-called Lightly Doped Drain (LDD), is provided between a channel region and a high-concentration impurity region. For example, as regards a thin-film transistor having two gate electrodes, there is known a structure in which an LDD region is provided between a first channel region and a second channel region, or a structure in which LDD regions are provided on both sides of each of a first channel region and a second channel region.